1. Field of the Invention
The invention relates to methods for determiing route angles. More particularly, this invention pertains to a method for determining route angles by means of and for the automatic calibration of a three-axis magnetometer that is fixedly mounted in an aircraft and is subject to interfering vehicle magnetic fields and instrumentation errors.
2. Description of the Prior Art
The three-axis magnetometer measures the earth's magnetic field. Magnetic field lines are oriented with respect to the magnetic north pole to determine course. The course determination process includes determining the vehicle's longitudinal axis.
Horizontal intensity H is obtained from the total intensity T and inclination I of the field lines with respect to the horizontal plane. That is, EQU H=T. cos I (1-1)
Similarly, vertical intensity V is derived as: EQU V=T. sin I (1-2)
The unit of magnetic field measure is Oe or A/m (Oerstedt; 1 Oe=79.58 A/m; 1 A/m=0.01257 Oe). The total intensity of the earth's magnetic field varies from 15 to 60 A/m, while the inclination varies from 0.degree. in the vicinity of the equator to 90.degree. at the magnetic north and south poles. In central Europe, for example, total intensity is approximately 40 A/m and inclination is 65.degree..
In navigation, the difference between the geographic and magnetic north directions is required. This difference, known as declination or magnetic variation, is (as are total intensity and inclination) dependent upon location. In the vicinity of the poles, such difference becomes quite large (up to .+-.180.degree. ), while in central Europe, for example, it varies from -10.degree. to +10.degree..
Declination is defined: EQU .delta.=.psi.-.psi..sub.m ( 1-3)
where .psi.=true route angle (geographic north)
.psi..sub.m =magnetic route angle (magnetic north)
The magnetic field of the earth is not constant in time. Rather, it is subject to both long-term (or secular) and short-term changes. Short-term changes result, for example, from atmospheric disturbances such as magnetic storms, thunderstorms and northern lights. Long-term disturbances result from changes in the earth's fluid core (convection currents).
The following average values are representative for central Europe: ##EQU1##
Local anomalies in the earth's magnetic field (e.g. ore deposits) also produce effects that interfere with magnetic course determination.
At the present time, the calibration of aircraft-fixed three-axis magnetometers requires the solution of a complex and poorly conditioned system of equations. Such equations can only be solved (if at all) in conjunction with the execution of specific extreme flight maneuvers.
The high degree of complexity of the system of equations generally mandates that calibration take place on a compensating turntable, with the various reference directions determined by means of a theodolite and integrated compass. In accordance with this (very costly) method, differences between the magnetic course and a reference direction are determined optomechanically. Reference points are attached to the aircraft at various positions on a circle and are compensated in either hardware or software. The calibration must be repeated not only upon exchange of the three-axis magnetometer, but also upon exchange or alteration of the magnetically active parts of the aircraft (such as a helicopter) or upon variation of the loading program.